Why Doesn't the Viewport Optix Denoiser Doesn't Work in 2.9?
With the latest (as of this writing)  alpha release of 2.9 you can indeed use optix and take advantage of viewport denoising in cycles with  both CPU and certain non RTX GPUs.
From the Cycles release notes

GPU Rendering

OptiX support is now enabled on all NVIDIA GPUs that support it,    which is Maxwell or higher (GeForce 700, 800, 900, 1000 series).

NVLink support for CUDA and OptiX. When enabled in the Cycles device    preferences, GPUs connected with an NVLink bridge will share
memory    to support rendering bigger scenes.

NVIDIA GPU rendering on macOS is no longer supported. Apple dropped    support for CUDA drivers in macOS 10.14, and no recent Apple hardware
uses NVIDIA graphics cards.





Viewport Denoising
Intel OpenImageDenoise is now supported for interactive denoising in
the 3D viewport. This works on any recent Intel/AMD CPU (with SSE
4.1).
Previously viewport denoising was only supported for NVIDIA GPUs using
OptiX.